Pull safe_conversions from chromium

Manual copy and paste of all code found in the namespace base. I didn't
change any of the implementation code. Pull in a new file for optimized
ARM implementation.

Added a list of adaptions made to document what is different from
chromium.

Change-Id: I88b4af45437506cf57755e48fdfc88027a5aed33
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2436610
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70452}

BUILD.gn

@@ -4065,6 +4065,7 @@ v8_component("v8_libbase") {
     "src/base/region-allocator.h",
     "src/base/ring-buffer.h",
     "src/base/safe_conversions.h",
+    "src/base/safe_conversions_arm_impl.h",
     "src/base/safe_conversions_impl.h",
     "src/base/small-vector.h",
     "src/base/sys-info.cc",

src/base/safe_conversions_arm_impl.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Slightly adapted for inclusion in V8.
+// Copyright 2014 the V8 project authors. All rights reserved.
+// List of adaptations:
+// - include guard names
+// - wrap in v8 namespace
+// - include paths
+
+#ifndef V8_BASE_SAFE_CONVERSIONS_ARM_IMPL_H_
+#define V8_BASE_SAFE_CONVERSIONS_ARM_IMPL_H_
+
+#include <cassert>
+#include <limits>
+#include <type_traits>
+
+#include "src/base/safe_conversions_impl.h"
+
+namespace v8 {
+namespace base {
+namespace internal {
+
+// Fast saturation to a destination type.
+template <typename Dst, typename Src>
+struct SaturateFastAsmOp {
+  static constexpr bool is_supported =
+      std::is_signed<Src>::value && std::is_integral<Dst>::value &&
+      std::is_integral<Src>::value &&
+      IntegerBitsPlusSign<Src>::value <= IntegerBitsPlusSign<int32_t>::value &&
+      IntegerBitsPlusSign<Dst>::value <= IntegerBitsPlusSign<int32_t>::value &&
+      !IsTypeInRangeForNumericType<Dst, Src>::value;
+
+  __attribute__((always_inline)) static Dst Do(Src value) {
+    int32_t src = value;
+    typename std::conditional<std::is_signed<Dst>::value, int32_t,
+                              uint32_t>::type result;
+    if (std::is_signed<Dst>::value) {
+      asm("ssat %[dst], %[shift], %[src]"
+          : [dst] "=r"(result)
+          : [src] "r"(src), [shift] "n"(IntegerBitsPlusSign<Dst>::value <= 32
+                                            ? IntegerBitsPlusSign<Dst>::value
+                                            : 32));
+    } else {
+      asm("usat %[dst], %[shift], %[src]"
+          : [dst] "=r"(result)
+          : [src] "r"(src), [shift] "n"(IntegerBitsPlusSign<Dst>::value < 32
+                                            ? IntegerBitsPlusSign<Dst>::value
+                                            : 31));
+    }
+    return static_cast<Dst>(result);
+  }
+};
+
+}  // namespace internal
+}  // namespace base
+}  // namespace v8
+
+#endif  // V8_BASE_SAFE_CONVERSIONS_ARM_IMPL_H_